The latest patch from Victor looks good.  A few comments:

(1) the number of bits should be computed first directly using C 
arithmetic, and only recomputed using PyLong arithmetic if the C 
computations overflow.  For one thing, overflow is going to be very rare 
in practice;  for another, in the sort of applications that use 
.numbits(), speed of numbits() is often critical.

(2) Just as a matter of style, I think "if (x == NULL)" is preferable
to "if (!x)".  At any rate, the former style is much more common in
Python source.

(3) the reference counting all looks good.

(4) I quite like the idea of having numbits be a property rather than a 
method---might still be worth considering?